ORIGIN OF NONLINEARITY OF VOLTAGE-CURRENT RELATIONSHIPS OF TURTLE CONES

The voltage-current relations of the cone membrane were measured by means of pulses of current injected into single cones of turtle retina. In darkness the voltage-current curve was almost linear. During illumination with a small (dia. 115 μm) spot the steepness of the curve increased but its linearity was preserved. However, during illumination with a big (dia. 2 mm) spot the positive pulses of current evoked considerably higher potential drops than negative pulses of the same intensity. Accordingly, the depolarizing portion of the voltage-current curve was steeper than the hyperpolarizing one. This non-linear behaviour of cone membrane displayed only with a large-area illumination can be explained as a combined effect of non-linearity of presynaptic membrane of the cone and of the mechanism of feedback between the horizontal cells and cones. The equivalent circuit of synapse between the photoreceptor and the horizontal cell, realized as a physical model, is described. The model reproduces the effect of area illuminated on the shape of the voltage-current curves of the cone, as well as some other experimental manifestations of the feedback mechanism in cones. © 1979.